Using these two pairs of cell lines, we examined the relationship between Ido1 expression and cancer cell growth in vitro and in vivo. Our analysis of tumor growth and metastasis, in immunocompetent and immunodeficient mice, revealed that IDO1 not only modulated the immunological system, but also played an important biological role in tumor cell proliferation, cell cycle regulation, and antiapoptotic signaling. The gene discussed is IDO1; the disease is neoplasm.